The present invention relates to optical passive components for use in an optical fiber amplifier.
In a known optical fiber amplifier, pumping light having a wavelength of 1480 nm, for example, is incident upon an erbium-doped optical fiber so as to excite erbium atoms to high energy level. When signal light having a wavelength in the vicinity of 1550 mn is incident upon this excited erbium-doped optical fiber, stimulated emission and radiation of an identical wavelength proportional to magnitude of the signal light occurs so as to amplify the signal light. In order to excite the optical fiber amplifier, it is possible to use a front pumping method in which the pumping light is incident upon the erbium-doped optical fiber in a direction identical with that of propagation of the signal light, a rear pumping method in which the pumping light is incident upon the erbium-doped Optical fiber in a direction opposite to that of propagation of the signal light or a bidirectional pumping method in which the pumping light is incident upon the erbium-doped optical fiber both in the direction of propagation of the signal light and in its opposite direction. The front pumping method is effective for reducing noise figure (NF) and the rear pumping method is suitable for a case in which high output should be obtained. Meanwhile, the bidirectional pumping method has performance intermediate between those of the front pumping method and the rear pumping method. These pumping methods are selectively used on the basis of specifications such as amplification output, input power, gain, NF, etc. and required performance.
The known optical fiber amplifier is basically constituted by a pumping light source, a wavelength optical multiplexer, a non-polarized light type optical isolator and an erbium-doped optical fiber. In the optical fiber amplifier, as input power of pumping light to the erbium-doped optical fiber is increased, the input signal is amplified further. Thus, in case high output should be obtained, polarized light coupling is performed by using two pumping light sources.
Conventionally, as proposed in a paper (C-262) entitled "Integrated Optical Module for Er-doped Fiber Amplifiers" in Spring Meeting of Japanese Society of Electronic Data Communication in 1992, seven Optical functions, i.e. a polarized light coupler, a wavelength optical multiplexer, an optical isolator, a band-pass filter (BPF) for cutting natural light, two kinds of non-polarized light couplers for monitoring and a photodiode are integrated as an optical passive component for use in an optical fiber amplifier for rear pumping. By polarized light coupling function of this prior art optical passive component, two pumping light beams proceeding at right angles are coupled by a polarized beam splitter (PBS) and the coupled light beam is reflected by a wavelength selecting filter so as to be outputted to the erbium-doped optical fiber. On the other hand, signal light amplified by the erbium-doped optical fiber is passed through the wavelength selecting filter so as to be outputted through such optical components as the optical isolator, the band-pass filter (BPF) and the first non-polarized light coupler for monitoring arranged sequentially. A portion of light, which is reflected by the first non-polarized light coupler for monitoring, is reflected by the second non-polarized light coupler for monitoring so as to be carried to the photodiode. Furthermore, a portion of light, which has been passed through the second non-polarized light coupler for monitoring, is fed, as a monitoring output, to the optical fiber.
However, since input and output optical fibers extend from the prior art optical passive component in opposite directions, a large area is required for mounting the prior art optical passive component including laying of the optical fibers. As a result, the known optical fiber amplifier employing this optical passive component also becomes large in size undesirably. Furthermore, since monitoring function is performed at the last stage including the optical isolator and the band-pass filter, it is not easy to discriminate between failure of the optical amplifier portion and failure of the optical isolator or the band-pass filter. In addition, if the known optical passive component is used for the front pumping method, structural design based on new layout of the optical components is required to be performed, which is uneconomical.